On the Origin of Fast Rotating Stars. I. Photometric calibration and results of AO-assisted BVRI+Halpha imaging of NGC330 with SAMI/SOAR

TL;DR

This study uses AO-assisted BVRI+Halpha imaging to identify and analyze the Be star population in NGC330, revealing mass stratification and a lower overall Halpha emitter fraction than previous estimates.

Contribution

It introduces a comprehensive methodology for detecting Halpha emitters across the entire B-type spectral range in the Magellanic Clouds, with detailed analysis of NGC330's stellar distribution.

Findings

Identified Halpha emitters across all B-type stars in NGC330.

Discovered a bimodal radial distribution indicating mass stratification.

Found a lower overall Halpha emitter fraction compared to previous studies.

Abstract

H$\alpha$ emission is a clear indicator of circumstellar activity in Be stars, historically employed to assess the classical Be star (CBe) population in young open clusters (YOCs). The YOC NGC330 in the Small Magellanic Cloud exhibits a large known fraction of {CBe} stars and was selected for a pilot study to establish a comprehensive methodology for identifying H$\alpha$ emitters in the Magellanic Clouds, encompassing the entire B-type spectral range. Using the SOAR Adaptative Module Imager (SAMI), we investigated the stellar population of NGC330 using multi-band BVRI+H$\alpha$ imaging. We identified H$\alpha$ emitters within the entire V-band range covered by SAMI/SOAR observations ($V\lesssim22$), comprising the complete B-type stellar population and offering a unique opportunity to explore the Be phenomenon across all spectral sub-classes. The stellar radial distribution shows a clear bimodal pattern between the most massive (B5 or earlier) and the lower-mass main-sequence objects (later than B6) within the cluster. The former is concentrated towards the cluster center (showing a dispersion of $\sigma=4.26\pm0.20$ pc), whereas the latter extends across larger radii ($\sigma=10.83\pm0.65$ pc), indicating mass stratification within NGC330. The total fraction of emitters is $4.4\pm0.5\%$, notably smaller than previous estimates from flux- or seeing-limited observations. However, a higher fraction of H$\alpha$ emitters is observed among higher-mass stars ($32.8\pm3.4\%$) than within lower-mass ($4.4\pm0.9\%$). Consequently, the putative CBe population exhibits distinct dynamical characteristics compared to the bulk of the stellar population in NGC330. These findings highlight the significance of the current observations in providing a complete picture of the CBe population in NGC330.